Relativity.

If all the flowers were roses, A rose would not be rare; If all the days were cloudless, We’d call not any fair. If all the books were bibles, But few would learn to read; If all the world were Christians, There’d be no better creed. If everything were scarlet, There’d be no black or white; If there were nothing evil, There’d be no wrong or right. If every zone were frigid, There’d be no milder clime; If everybody pilfered, To steal would be no crime. If people did not differ In sentiment and thought, Mankind would be a unit, And life would be for naught.